This invention relates generally to millimeter wavelength, frequency scannable dielectric antennas, and more particularly, to controlled aperture illumination of such antennas.
Antenna beam scanning can be accomplished by several methods depending on the requirements of a particular application. In the case of microwave frequency applications, the simplest expedient is often found to be that of mechanically scanning the antenna by physically moving the entire antenna structure. For system applications where fast and precise beam steering is required, inertialess beam scanning is used. Inertialess beam scanning is generally accomplished electronically by altering the phase of a traveling wave across the radiating aperture of a waveguide using discrete phase-shifting components or by altering the frequency whereby an inherent phase shift is attained between individual radiating elements.
For millimeter wave frequencies, that is, the 30 to 300 GHz range, mechanical scanning is used in virtually all system applications. This is a result of the fact that inertialess scanning at millimeter wavelengths has been difficult to achieve because of the impracticality in size of the components that would be needed for beam steering.
Recent advancements in the field of millimeter wave antennas have resulted in the development of an inertialess scanning device in the form of the dielectric waveguide line source antenna as disclosed in U.S. Patent Application Ser. No. 409,201, now issued as U.S. Pat. No. 4,468,673. This type of antenna is a travelling wave type of structure and is unique in that the transmission line and the antenna aperture are an integral, homogeneous structure having radiation characteristics derived by way of the introduction of a number of identical slots cut into one wall of the transmission line. For the case in which all of the radiating slots are identical, the antenna displays a radiation pattern characterized by high, close-in sidelobes on the order of 12 to 13 dB. Sidelobes of this order are often found to be unacceptable for high performance radar and communications systems.
In order to reduce these high sidelobes, a symmetrically tapered amplitude distribution is required That is, a greater amount of energy should be radiated from the center of the array of radiating elements as compared to those elements at the ends. This type of distribution may be achieved by varying the conductance of the radiating elements along the length of the array.
In the case of a metallized antenna structure, the thickness of the metallized radiating elements along the array can be varied in order to produce the desired tapered amplitude excitation. This approach to the problem, however, offers no solution in the case of a dielectric antenna system.